LAS VEGAS, April 26 (UPI) — Not all solar thermal power plants, touted for their environmental benefits, are pollution-free because they also use other fuels, U.S. experts note.

Solar thermal plants are expected to sprout up across the sunny U.S. Southwest as the nation looks for so-called green energy alternatives. But the Las Vegas Sun reported Monday some of the solar plants are designed to use natural gas or propane boilers to jump-start the units each day, releasing greenhouse gases.

While the solar plants would produce only a small fraction of the carbon monoxide, nitrogen oxide and particulates natural gas-fired power plants produce, the amounts would add up for a region in which no fossil-fuel plants are planned, the Sun said.

About 60 solar power projects are on the drawing boards for federal land in southern Nevada, though U.S. Bureau of Land Management records don’t indicate which would use boilers. The Sun said 32 are identified as either solar thermal or concentrating solar power, which includes both solar thermal and advanced solar photovoltaic technologies.

The newspaper said the U.S. Environmental Protection Agency says the impact of a large number of solar thermal power plants on the region can’t be determined since different technologies are used. But some environmental activists in the state stated concern that southern Nevada’s air quality could get worse, the Sun said.